1. Field of the Invention
The present invention relates to an objective lens assembly, and to an optical head and an optical recording/reproduction apparatus using the objective lens assembly. Specifically, the present invention relates to an objective lens assembly having a high numerical aperture (NA) which is used for optical information processing, optical transmission, etc., and to an optical head and an optical recording/reproduction apparatus using such an objective lens assembly.
2. Description of the Related Art
In recent years, a digital versatile disc (DVD) has been receiving attention as a large storage optical medium because of its high recording density for recording digital information, which is six times higher than that of a compact disc (CD). Along with the increase of information to be stored, an optical recording medium which has a greater recording density than the DVD has been demanded. In order to achieve a higher recording density than the conventional DVD (wavelength: 660 nm, numerical aperture (NA): 0.6), it is necessary to shorten the wavelength of a light source and increase the NA of an objective lens assembly. For example, when using a blue laser which operates at a wavelength of 405 nm as a light source and an objective lens having a NA of 0.85, a recording density achieved is five times greater than that of the above DVD.
Such a high numerical aperture cannot be achieved by a single lens. Thus, an objective lens assembly structured by assembling a plurality of lenses is used in order to obtain a high numerical aperture. However, variations which may occur in the production of respective component lenses of the lens assembly cause aberrations. In order to correct such aberrations, there are various techniques proposed.
Japanese Laid-Open Publication No. 11-203706 proposes a technique for adjusting the relative positions of respective component lenses by moving the component lenses in order to correct aberration. FIG. 12 shows a structure of a conventional objective lens assembly 80 described in Japanese Laid-Open Publication No. 11-203706. The objective lens assembly 80 includes a first lens 81, a second lens 82, a first lens holder 83 for holding the first lens 81, and a second lens holder 84 for holding the second lens 82. The first lens holder 83 has the shape of a hollow cylinder. The first lens holder 83 has an end face 83A which faces the second lens holder 84. In the end face 83A, a cylindrical opening 83B is formed. At a central portion of a bottom face 83C of the opening 83B, the first lens holder 83 has a hole 83D for holding the first lens 81. The first lens 81 is fit into the hole 83D such that the optical axis thereof is generally perpendicular to the end face 83A. The second lens holder 84 also has the shape of a hollow cylinder. The second lens holder 84 has an end face 84A which faces the end face 83A of the first lens holder 83 and an end face 84E which is opposite to the end face 84A. In the end face 84E, the second lens holder 84 has a cylindrical opening 84B whose diameter is larger than that of the opening 83B. At a central portion of a bottom face 84C of the opening 84B, the second lens holder 84 has a hole 84D which has substantially the same diameter as that of the opening 83B of the first lens holder 83. The second lens 82 is retained in the opening 84B such that the optical axis thereof is generally perpendicular to the end face 84A.
In the objective lens assembly 80 having such a structure, in order to correct aberrations which may be caused by variations in production, relative positions of the first lens 81 and the second lens 82 are adjusted. Specifically, the relative position of the second lens 82 with respect to the first lens 81 is adjusted by adjusting the relative position of the second lens holder 84 with respect to the first lens holder 83 along the direction indicated by arrow X such that the optical axis of the second lens 82 coincides with the optical axis of the first lens 81. Then, the second lens 82 which is retained by a, spring (not shown) is moved along the direction indicated by arrow Y by turning a screw (not shown), whereby the relative position of the second lens 82 with respect to the first lens 81 along the direction indicated by arrow Y is adjusted. Thereafter, the first lens holder 83 and the second lens holder 84 are adhered to each other such that the end face 83A faces the end face 84A. In this way, the relative position of the second lens 82 with respect to the first lens 81 is adjusted such that aberration of each component lens is corrected. As a result, an objective lens assembly having a high NA can be obtained.
Japanese Laid-Open Publication No. 2000-90473 proposes a structure where component lenses are incorporated into lens holders without strictly adjusting the relative positions of the component lenses, and one of the incorporated lenses is polished so as to correct aberration. FIG. 13 shows a structure of a conventional objective lens assembly 90 described in Japanese Laid-Open Publication No. 2000-90473. The objective lens assembly 90 includes a first lens 91, a second lens 92 integrally formed from a plurality of single-lenses, and a lens holder 93 for holding the first lens 91 and the second lens 92. The lens holder 93 has the shape of a hollow cylinder and has end faces 93A and 93B. The lens holder 93 has an opening 93C which penetrates through the end faces 93A and 93B and which has steps inside thereof. The first lens 91 is held in the opening 93C at the end face 93A. The second lens 92 is held inside of the opening 93C. The first lens 91 has an end face 91A on the opposite side from the second lens 92 in relation to the first lens 91.
The manner in which the objective lens assembly 90 having such a structure is assembled is described. In the first step, the first lens 91 and the second lens 92 are incorporated in the lens holder 93 without strictly adjusting the relative positions of these lenses. Next, the end face 91A of the first lens 91 is polished such that a total aberration of the first lens 91 and the second lens 92 is reduced, thereby completing the objective lens assembly 90. Specifically, aberrations caused due to variations in production of each of the first lens 91 and the second lens 92 and aberrations caused due to variations which may occur when incorporating the first lens 91 and the second lens 92 in the lens holder 93 are corrected by generating aberration by polishing the end face 91A of the first lens 91. Thus, when an objective lens assembly is assembled by using this method, a wider production tolerance for each lens component and a wider incorporation tolerance for incorporation into a lens holder can be provided.
Japanese Laid-Open Publication No. 11-174307 proposes an objective lens assembly where two lens holders each incorporating a lens are combined such that one is fit into the other, and the interval between these two lenses can be adjusted.
In adjustment of an objective lens assembly including two lens holders, one of the lens holders is moved with respect to the other such that the relative positions of these two lenses are changed. In this case, it is desirable to move one of the lens holders along five axes (lens interval direction, two directions for lens tilt, and two directions for adjusting the center of the lens), such that the total aberration of the entire objective lens assembly is minimized. One of the known methods for measuring aberration is a method using a Twyman interferometer. A conventional adjustment method for adjusting the relative positions of the two lens holders while measuring the aberration with the Twyman interferometer is described with reference to FIG. 14.
FIG. 14 shows a structure of a conventional objective lens assembling/adjusting apparatus 1400. Light is supplied to a half mirror 101 in the direction indicated by arrow 145 in order to measure the total aberration of an entire objective lens assembly. Some components of the supplied light are reflected by the half mirror 101, and the other components of the supplied light reach a second lens 22 of an objective lens assembly 144 which are measured. Some components of the light reflected by the half mirror 101 are further reflected by a reflection mirror 112 and are transmitted through the half mirror 101 so as to reach a detection panel 111. On the other hand, the other components of the supplied light which have reached the second lens 22 of the objective lens assembly 144 are transmitted through a first lens 21 of the objective lens assembly 144 so as to enter a reference sphere 106. The reference sphere 106 is positioned such that the center of the reference sphere 106 coincides with a focal point 107 of the objective lens assembly 144. Light concentrated on the focal point 107 crosses the surface of the reference sphere 106 at right angles. The light entering into the reference sphere 106 toward the focal point 107 is reflected by the surface of the reference sphere 106 and returns via the same route. This reflected light is further reflected by the half mirror 101 so as to be incident on the detection panel 111.
The light reflected by the reflection mirror 112 and the light reflected by the reference sphere 106 generate an interference pattern 113 on the detection panel 111. Based on this interference pattern 113, the aberration which may be caused by a combination of the first lens 21 and the second lens 22 can be measured.
In the structure described in Japanese Laid-Open Publication No. 11-203706 shown in FIG. 12, a screw is used to adjust the relative position of the second lens 82 with respect to the first lens 81 along the direction indicated by arrow Y. This is not suitable for mass production of objective lens assemblies.
In the structure described in Japanese Laid-Open Publication No. 2000-90473 shown in FIG. 13, it is necessary to polish the end face 91A of the first lens 91 so as to obtain a desired surface shape. Such a polishing process is not suitable for mass production of objective lens assemblies.
In the structure described in Japanese Laid-Open Publication No. 11-174307, since the positions of the lenses are adjusted while one of the two lens holders fits into the other, adjustments of decentering and tilting are limited. In this structure, among various aberrations of each lens which may be caused by variations in production, only spherical aberration can be corrected, but comatic aberration and astigmatism cannot be corrected.
In such a case, a relative position (decenter or tilt) of each lens is adjusted before it is incorporated into a lens holder, and such an adjusted lens is adhered to a lens holder, whereby every aberration can be corrected. In this case, however, it is necessary to strongly adhesively fix the lens to the lens holder.
Such a strong adherence results in a stress being applied to each lens. Furthermore, when an adhesive agent is applied to a curved lens surface, the amount of the adhesive agent between the lens and the lens holder becomes non-uniform. In such a case, if the temperature is increased, the relative positions of the lens and the lens holder significantly changes, and as a result, the temperature characteristic of the objective lens assembly is deteriorated. This problem may be caused not only when first and second lenses whose relative positions are adjusted are respectively adhered to first and second lens holders, but also when the first and second lenses whose relative positions are adjusted are adhered to a single lens holder.
Furthermore, in the structure of the conventional objective lens assembling/adjusting apparatus 1400 in FIG. 14, when the relative positions of the first lens 21 and the second lens 22 are changed, the position of the focal point 107 is also changed. Thus, it is necessary to move the reference sphere 106 according to the change in position of the focal point 107 such that the center of the reference sphere 106 coincides with the focal point 107. Moreover, there are three directions (X-axis direction, Y-axis direction, and Z-axis direction) in which the reference sphere 106 is to be moved. Therefore, it is necessary to detect offset information as to the positional offset between the center of the reference sphere 106 and the focal point 107 for each of the three directions, and it is necessary to move the reference sphere 106 based on a detection result such that the center of the reference sphere 106 coincides with the focal point 107. Such a complicated procedure increases the number of steps in assembling an objective lens assembly, and the increase in the number of assembly steps becomes an obstacle to the mass production of objective lens assemblies.
When measuring aberration by such an interferometer based on the change of the two-beam interference pattern 113, the flow of air between the objective lens assembly 144 and the reference sphere 106 is turbulent, and such turbulence of an air layer causes waves in the interference pattern 113. When waves in the interference pattern 113 are caused, the aberration of the objective lens assembly cannot be measured with high accuracy, and therefore, the objective lens assembly cannot be assembled with high accuracy.
According to one aspect of the present invention, an objective lens assembly includes: at least two groups of lenses having a common optical axis; and at least two lens holders for respectively holding the at least two groups of lenses, wherein each of the lens holders has an end face perpendicular to the optical axis, and the lens holders are positioned such that the end face of one of the lens holders faces the end face of the other lens holder, and a gap between these end faces is filled with an adhesive agent for adhering the lens holders to each other.
In one embodiment of the present invention, a numerical aperture (NA) of the objective lens assembly is 0.7 or more.
In another embodiment of the present invention, chromatic aberration of the objective lens assembly for light having a wave length from 390 nm to 450 nm is corrected.
In still another embodiment of the present invention, one of the at least two lens holders is adjusted with respect to at least another one of the at least two lens holders along at least five directions including a direction parallel to the optical axis, two directions perpendicular to the optical axis, and two tilt directions, and thereafter is adhered by the adhesive agent to the at least another one of the at least two lens holders.
In still another embodiment of the present invention, one of the at least two lens holders is adjusted with respect to at least another one of the at least two lens holders along at least five directions including a direction parallel to the optical axis, two directions perpendicular to the optical axis, and two tilt directions, such that aberration of the objective lens assembly is equal to or smaller than a predetermined value.
In still another embodiment of the present invention, the predetermined value of the aberration of the objective lens assembly is about 70 mxcex.
In still another embodiment of the present invention, a shape of at least one of the at least two lens holders is different from those of the other lens holders of the at least two lens holders.
In still another embodiment of the present invention, all of the at least two lens holders have the same shape.
In still another embodiment of the present invention, the at least two lens holders are all formed of the same material.
In still another embodiment of the present invention, a material used for forming at least one of the at least two lens holders is different from that used for forming the other lens holders of the at least two lens holders.
In still another embodiment of the present invention, at least one of the at least two lens holders has a mirror face at a position opposite to the end face thereof.
In still another embodiment of the present invention, the mirror face is formed by plating or vapor deposition.
In still another embodiment of the present invention, at least one of the at least two lens holders has an aperture formed therein for determining a size of the numerical aperture (NA) of the objective lens assembly.
In still another embodiment of the present invention, the lens holder is blackened or made of a transparent material such that reflectance inside the lens holder is reduced.
In still another embodiment of the present invention, a group of lenses among the at least two groups of lenses are contained inside the lens holder for holding the group of lenses.
In still another embodiment of the present invention, at least one group of the at least two groups of lenses partially protrudes from at least one of the at least two lens holders for holding the at least two groups of lenses.
In still another embodiment of the present invention, a material for forming the at least two lens holders is a metal.
In still another embodiment of the present invention, a material for forming the at least two lens holders is a resin.
Instill another embodiment of the present invention, the thermal expansion coefficient of the resin is isotropic.
In still another embodiment of the present invention, at least one group among the at least two groups of lenses and at least one of the at least two lens holders for holding the one group among the at least two groups of lenses are integrally formed.
According to another aspect of the present invention, an objective lens assembly includes: at least two groups of lenses having a common optical axis; and at least two lens holders for respectively holding the at least two groups of lenses, wherein at least one of the at least two lens holders has an engagement portion for engaging with an actuator which drives the objective lens assembly such that the position of the objective lens assembly is controlled.
In one embodiment of the present invention, each of the lens holders has an end face perpendicular to the optical axis; and the lens holders are positioned such that the end face of one of the lens holders faces the end face of another lens holder, and a gap between these end faces is filled with an adhesive agent for adhering the lens holders to each other.
In another embodiment of the present invention, the engagement portion is provided at a position corresponding to a plane which includes the centroid of the objective lens assembly and is vertical to the optical axis.
According to still another aspect of the present invention, an apparatus for assembling/adjusting an objective lens assembly of claim 5 includes: a light source for emitting light toward the objective lens assembly; a diffraction grating for generating interference fringes based on the light transmitted through the objective lens assembly; an aberration calculation section for calculating aberration of the objective lens assembly based on the interference fringes; and a driving section for driving one of the at least two lens holders to move with respect to another one of the lens holders along at least five directions including a direction parallel to the optical axis, two directions perpendicular to the optical axis, and two tilt directions, such that the aberration of the objective lens assembly which is calculated by the aberration calculation section is equal to or smaller than the predetermined value.
In one embodiment of the present invention, the diffraction grating is formed on a glass substrate; and the thickness of the glass substrate is determined such that the glass substrate has an aberration equal to that of an optical recording medium on which an optical recording/reproduction apparatus incorporating a completed objective lens assembly performs recording and/or reproduction.
In another embodiment of the present invention, the diffraction grating separates the light transmitted through the objective lens assembly into +1st-order diffraction light, xe2x88x921st-order diffraction light, and transmitted light; and the apparatus for assembling/adjusting an objective lens assembly further includes a numerical aperture management section for managing the size of aperture for determining the size of a numerical aperture of the objective lens assembly based on a positional relationship between a first overlapping area of the +1st-order diffraction light and the transmitted light and a second overlapping area of the xe2x88x921st-order diffraction light and the transmitted light.
According to still another aspect of the present invention, an optical head for recording or reproducing a signal on an optical recording medium includes: a light source; and the objective lens assembly of claim 1 located between the light source and the optical recording medium.
In one embodiment of the present invention, the optical head further includes an actuator which includes a movable member coupled to the optical lens assembly, a movable member holding section for holding the movable member, and a movable member driving section coupled to the movable member holding section for driving the movable member, wherein the movable member included in the actuator and the at least two lens holders are formed of the same material.
In another embodiment of the present invention, the optical head further includes an actuator which includes a movable member coupled to the optical lens assembly, a movable member holding section for holding the movable member, and a movable member driving section coupled to the movable member holding section for driving the movable member, wherein the movable member included in the actuator functions as at least one of the at least two lens holders.
According to still another aspect of the present invention, an optical head for recording or reproducing a signal on an optical recording medium includes: a light source; and the objective lens assembly of claim 21 located between the light source and the optical recording medium.
In one embodiment of the present invention, the optical head further includes an actuator which includes a movable member coupled to the optical lens assembly, a movable member holding section for holding the movable member, and a movable member driving section coupled to the movable member holding section for driving the movable member, wherein the engagement portion of one of the at least two lens holders engages with the movable member included in the actuator.
In another embodiment of the present invention, the engagement portion is provided such that the centroid of the objective lens assembly coincides with a center of driving of the actuator.
According to still another aspect of the present invention, an optical recording/reproduction apparatus for recording or reproducing a signal on an optical recording medium includes: a motor for rotating the optical recording medium; the optical head of claim 27; and a processing circuit for controlling the motor and the optical head.
According to still another aspect of the present invention, an optical recording/reproduction apparatus for recording or reproducing a signal on an optical recording medium includes: a motor for rotating the optical recording medium; the optical head of claim 30; and a processing circuit for controlling the motor and the optical head.
Thus, the invention described herein makes possible the advantages of (1) providing an objective lens assembly where aberration of each component lens which may be caused due to variations in the production thereof can be corrected by a method suitable for mass production of the objective lens assembly, and an optical head and an optical recording/reproduction apparatus using such an objective lens assembly, (2) providing an objective lens assembly having a superior temperature characteristic, and an optical head and an optical recording/reproduction apparatus using such an objective lens assembly, and (3) providing an objective lens assembly where aberration of each component lens which may be caused due to variations in the production thereof can be corrected with high accuracy, and an optical head and an optical recording/reproduction apparatus using such an objective lens assembly.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.